Secondary battery with movable shutter means between fixed electrodes

ABSTRACT

An electrically regenerable electrochemical system has a rotating apertured shutter between fixed negative and positive electrodes which provides both electrolyte circulation and pulsed charging to form deposits on the electrodes during the charge mode of the repeated charge and discharge cycles that are smooth, uniform and adherent. A zinc-nickel oxide secondary battery of this system is uniquely suited for use in hybrid vehicular power systems because of its ability to provide long cycle life at very high power densities under conditions of rapid cycling.

United States Patent 1191 McCoy 1 Oct. 2, 1973 1 1 SECONDARY BATTERYWITH MOVABLE SHUTTER MEANS BETWEEN FIXED ELECTRODES [75] Inventor:Lowell R. McCoy. Woodland Hills,

Calif.

[73] Assignee: Rockwell International Corporation, El Segundo, Calif.

221 Filed: Mar. 29, 1971 21 Appl. No.: 129,089

[52] U.S. Cl.

136/141 511 Int. Cl. ..n01m 31/02 [58] Field 6: Search 136/141,140,159-161, I 136/6, 28, 30, 86, 75-76 [56] 1 References Cited Y UNITEDSTATES PATENTS 543,680 7/1895 Epstein 136/141 3,440,098 4/19 9Stachurski' 136/6 717,395 12/1902 Halsey 136/140 734,547 7/1 0 Halsey136/141 3,257,241 6/1966 Tamminen 136/90 3,275,475 9/1966 Cohn etal......,.. 136/86 E 3,359,136 12/1967 Mcrten et al. 136/30 X 3,560,2612/1971 Stachurski ct a1. 136/141 Halsey 136/140 Primary Examiner-AnthonySkapars Attorney-L. Lee Humphr ies, Thomas S. MacDonald and Henry Kolin[57] ABSTRACT An electrically regenerable electrochemical system has arotating apertured shutter between fixed negative and positiveelectrodes which provides both electrolyte circulation and pulsedcharging to f orm deposits on the electrodes during the charge mode ofthe repeated charge and discharge cycles that are smooth, uniform andadherent. A zinc-nickel oxide secondary battery of this system isuniquely'suited for use in hybrid vehicular power systems because ofits'ability t0 provide long cycle life at very 'high power densitiesunder conditions of rapid cycling.

7 Claims, 2 Drawing Figures PAIENHD 2 INVENTOR.

LOWELL R. MC COY I ATTORNEY SECONDARY BATTERY WITH MOVABLE SHUTTER MEANSBETWEEN FIXED ELECTRODES BACKGROUND OF THE INVENTION This inventionrelates to an improved electrically regenerable electrochemical system.It more particularly relates to a zinc-nickel oxide secondary batteryproviding long cycle life at very high power densities under conditionsof rapid cycling.

The present electrically regenerable electrochemical system byprovidinga long cycle life at high power densities finds particularutility in a hybrid vehicular power system. Such a hybrid systemconsists of a conventional heat engine, such as an internal combustionengine, coupled with a secondary battery, and is intended tosubstantially reduce the amount of air pollutants emitted when a heatengine alone is used as power source. The battery used in a hybridsystem needs to store only a relatively small amount of energy forutilization dur' ing limited periods of high-power demand, such asduring vehicle acceleration. Thus the energy-densityrequirements forbatteries for use in a hybrid vehicle are less critical than those ofbatteries required for use in all-electric vehicles. On the other hand,rapid cycling at high current levels in charge and discharge modes ofbut several seconds or minutes duration requires that the batteries usedfor hybrid systems possess very long cycle lives of at least severalthousand cycles and provide exceptionally high power densities in excessof 100 watts per pound. While an improved electrically regenerableelectrochemical system may be made utilizing a large variety ofdifferent negative and positive electrodes cooperating with the movableshutter means, this system will be particularly described with respectto an improved secondary battery using a zinc-nickel oxide alkalineelectrochemical system because of the particular utility of such abattery in a hybrid vehicle.

The unique position of zinc as a negative electrode for use in electriccells and batteries has long been recognized because zinc is cheap,abundant, has a low rate of self-discharge in alkaline electrolytes, andhas a high energy density when used in combination with commoncounterelectrodes. For these reasons, it is used extensively in primarybatteries. It is also attractive for use in secondary batteries as itprovides the highest energy density of the metals that can beelectrodeposited from an inexpensive, highly conductive aqueouselectrolyte at ambient temperatures. However, the use of zinc electrodesin secondary batteries has been severely limited by their failure towithstand repeated cycling at high charge and discharge rates and by thelimited depth of discharge that can be achieved without an ir reversibleloss of capacity. In particular, repeated recharge of these zincelectrodes leads to undesirable changes in the electrode structure. Thusa frequent mode of failure has consisted in the formation of nonadherentmossy and dendritic deposits instead of smooth adherent deposits. Thenonadherent mossy deposit, which is black, fluffy and porous, is formedat low overpotentials; whle the dendritic deposits are formed at highoverpotentials.

Many attempts have been made to overcome these problems encountered inthe attempted use of zinc electrodes in secondary batteries. Some ofthese approaches have involved utilizing a porous electrode structure,circulating the electrolyte, and using rotating planar electrodes. See,for example, U. S. Pat. Nos. 3,359,136; 3,275,475; and 3,440,098 inwhich are described some of the problems encountered in using anelectrochemically reversible electrode, particularly a zinc/zinc oxideelectrode subjected to charge-discharge cycling. ln U. S. Pat. No.3,440,098, it is proposed to overcome the development of dendriticdeposits, which tend to bridge the interelectrode gap between thereversible electrode and the counterelectrode thereby shorting the cell,by using a rotating wiper member for sweeping the face of the activeelectrode. It has also been proposed to use an electrically pulsedcurrent source or periodic reversal of current during the chargingcycle.

While the foregoing techniques may provide partial solutions for one oranother of the various problems en countered in devising highperformance electrodes suitable for use in secondary batteries, to datethey have been unable to provide secondary batteries of commercialinterest that are capable of extended cycling at high charge-dischargerates. The electrically regenerable electrochemical system provided bythis invention, particularly when utilized as a rechargeable zinc-nickeloxide alkaline secondary cell or battery, provides for uniform andadherent deposition of the electrode materials during charge anddischarge cycles and is capable of rapid cycling and long cycle life atvery high power densities.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a unique electrically regenerable electrochemical system thathas a long cycle life at high power densities. It is a further object ofthe invention to provide an improved zincnickel oxide secondary batteryof particular utility in a hybrid vehicular power system.

In accordance with this invention, an electrically regenerableelectrochemical system is provided wherein a nonconductive movableshutter means is disposed be tween, but not in contact with, fixednegative and positive electrodes immersed in an electrolyte. The shuttermeans is in continuous movement during operation of the cell during bothcharge and discharge cycles to provide stirring or circulation of theelectrolyte and also to subject anygiven point on the electrode surfaceto a mechanical equivalent of pulsed charging, i.e., to alternatingperiods of maximum and minimum current density. Thereby uniform andadherent deposits are formed on the electrodes during the charge modeand electrode passivation is avoided during the discharge mode toprovide a cell capable of undergoing long cycle life at high" powerdensities.

In the preferred aspects of the invention, the nonconductive movableshutter means consists of a thin, rotating apertured or slotted disk.The apertures or slots generally consist of two or more open sectors,preferably having between 30 and per cent open area so as toprovide'minimal resistance to the flow of current through theelectrolyte during cell operation while still providing electrolyteagitation and pulsed-current charging. The rotational speed of theapertured disk is varied over wide limits, between 20 and 200 rpm,depending upon the ratio of open to closed spaces present in the disk.While any portion of either electrode may receive alternating periods ofhigh and low current den sities as low as one cycle per second to 10cycles per second without loss of efficiency, it is generally moreconvenient and preferred to operate the cell to provide between and 200cycles per second. Representative hybrid vehicular power systemsrequirements call for a 10- to 30-second charge cycle, with a dischargecycle as low as 6 seconds and as high as 12 minutes. While rotationalspeeds up to 200 rpm may be utilized, from a practical standpoint it ispreferred to keep the rotational speed as low as possible commensuratewith providing adequate movement of the electrolyte so as to minimizethe power losses involved in rotating the disk.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded schematic viewin perspective of a cell made in accordance with the present invention.

FIG. 2 is a cross-sectional schematic view of a secondary zinc-nickeloxide alkaline battery consisting of a battery module equivalent toeight electrode pairs connected in parallel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In its broadest aspects, theelectrically regenerable electrochemical system of this invention may beadvantageously utilized for many battery applications, particularlythose requiring rapid cycling and long cycle life at very high powerdensities. This long cycle life is provided because of the smooth,uniform and adherent deposits formed on the electrodes during the chargemode. Such deposits are obtained by moving a nonconductive movableshutter means between the stationary negative and positive electrodes sothat during operation of the cell, movement of the shutter means resultsin agitation of the electrolyte together with pulsed charging.

A wide variety of nonconductive movable shutter means may be utilized soas to expose portions of the fixed electrodes to alternating periods ofvarying current density during operation of the cell while at the sametime providing circulation of the electrolyte. Thus a slotted planarmask may be oscillated in an upward and downward movement, with slots inthe mask transverse to the direction of movement. However, such ashutter means would involve greater mechanical complexity since it wouldrequire changing direction of the moving mass at a desired frequency.Thus a thin, apertured or slotted rotating disk is preferred because ofits simplicity of construction and the minimal power requirements forrotating the disk in a single direction at a given frequency while atthe same time providing the desired electrolyte agitation and pulsedcharging of the electrodes. In general, the particular type andarrangement of apertures in the disk in the form of variously shapedslots, slits, holes or sectors is not considered critical, provided theamount of closed area of the rotating disk is not such as to undulyincrease the internal resistance of the cell. Conversely, the amount ofopen area should not be such as to interfere with providing an effectivemechanical equivalent of pulsed charging. In general, the open area willvary from 30 to 80 per cent of the total disk area. The apertures aregenerally and preferably arranged in the form of symmetrical sectors,anywhere from two to 10 such sectors being convenient and suitable.Thereby a uniform periodicity of the pulsed charging is obtained. Therotational speed of the apertured disk will of course be a significantfactor in determining the rate of cyclical charging of the cell.

The total rate at which an electrochemical process takes place at anelectrode surface is the product of the electrode surface area and therate per unit area (current density). Because smooth, uniform andadherent deposits are formed on the stationary electrodes when utilizingthe rotating shutter, it is feasible and desirable to increase theeffective surface area by utilizing highly porous substrates on whichthe active electrode material is deposited. Thus metallic screens andporous plaques may be advantageously utilized as substrates so that fora given electrode surface area a higher effective current density isobtained during the charge and discharge cycles, thereby providing aconsiderable increase in the power density of the cell.

A wide variety of electrodepositable metals, e.g., Zn, Cd, Sn, Pb, maybe employed as the active materials of the negative electrodes in thesecondary cell. During the discharge cycle of the cell, these metals ofthe nega tive electrode are oxidized. On recharging the cell, thesemetals are redeposited on the negative electrode. As used herein, theterm battery generally designates an assembly of substantially identicalunits or cells, but it may conveniently be used to designate only asingle unit. Similarly, use of the term counterelectrode merely refersto an opposing electrode, whether utilized as a negative or positiveelectrode or as an anode or cathode.

Because of the relatively high energy density and power density that maybe obtained with zinc, it is particularly preferred for use as thenegative electrode of an alkaline secondary cell. Such an electrode maybe employed with a wide variety of counterelectrodes, which may bechemically inert or be composed of electrochemically reversiblematerials. When the negative zinc electrode is used as an anode(discharge cycle) in a secondary cell, three cathodic materials are ofparticular interest for use therewith as counterelectrodes, namely,silver oxide, nickel oxide, and oxygen (or air). The conventionalalkaline electrolyte used in such cells is generally between 20 and 40wt. percent KOH.

The cell reaction for each of the foregoing systems can be written inidealized form as follows:

The foregoing secondary battery systems are well known and each offerscertain advantages and disadvantages under conditions of repeatedcycling in terms of loss of active material, chemical irreversibility,obtainable energy and power density, cost, and reliability. Use of therotating apertured shutter, particularly with respect to the zincelectrode, will provide an improvement for each of these secondary cellsystems, particularly in providing improved cycle life under conditionsof high current charge and discharge.

For purposes of illustration, because of its commercial importance inproviding long cycle life at very high power densities, the inventionwill be particularly described in its various preferred embodiments withreference to a secondary cell consisting of the zinc-nickel oxidesystem, although clearly not limited thereto.

Referring to FIG. 1, an exploded schematic view in perspective of anelectric cell 10 is shown, illustratively, a zinc-nickel oxide secondarycell. The insulated cell casing consists of end plates 12 and 14,suitably of polystyrene, with a gasket 16, suitably of rubber, used as aspacer to provide the necessary cell cavity. A positive nickel oxideelectrode 18 is suspended loosely within the cell by its externalcurrent lead (not shown). A negative electrode 20 consists of a coatingof zinc electrodeposited on a cadmium-plated copper screen. The zincelectrode support is affixed to cell end plate 12 by a suitable cement.

It is important that the negative electrode 20 be constructed orsurfaced with a metal capable of receiving an adherent plate of themetal to be electrodeposited. For example, the deposition of zinc uponnickel generally produces nonadherent deposits if the nickel has beensubjected to anodic treatment prior to such deposition. Thesenonadherent deposits may flake off or blister from the nickel causingshorting. Thus an underlying layer of cadmium is preferred for use as asubstrate on which zinc is deposited to improve adherence of the zinc.In addition, it is preferred that negative electrode 20 be in perforatedform, such as a screen. This improves the adherence of the depositedmetal, increases the effective surface area, and provides a saving inweight.

Because of the loose suspension of the nickel oxide electrode 18, a-mesh nylon screen 22 is used to pre vent direct contact between thenickel oxide electrode 18 and a rotating shutter 24. Screen 22 isgenerally of the same size, or slightly smaller, than electrode 18.Shutter 24 is in the form of an apertured disk, is made of a thininsulating plastic material, such as a highimpact-strength polystyrene,and is of the same of slightly larger diameter than electrodes 18 and20. As shown, shutter 24 has three apertured sectors which provide anopen area equal to that of the masked area. An annular spacer ring 26and an inner spacer disk 28, both made ofa suitable insulating plasticmaterial such as nylon, are used to prevent contact between the rotatingshutter 24 and the negative zinc electrode 20. Shutter 24 is fixedlymounted on a shaft 30 which extends through end plate 12 of the cellthrough a shaft bearing and seal 32, the shaftthen being connected to afixedspeed motor (not shown). Negative and positive current-conductingtabs 34 and 36, respectively, extend from the electrodes over the top ofthe cell walls and are attached to their respective lead wires (notshown).

In FIG. 2 is shown a cross-sectional schematic view of a secondaryzinc-nickel oxide alkaline battery 40, The battery module shown isequivalent to eight pairs of zinc and nickel oxide electrodes connectedin paral lel. In order to maximize the power density available,lightweight materials of construction are used throughout. The cellcasing 42 is preferably of polystyrene or polypropylene. The three innerzinc electrodes 44 are double-faced electrodes having zinc coatings onboth sides. The outer zinc electrodes 46, which are attached to theopposite side walls of casing 42, are single-faced electrodes. Thesenegative zinc electrodes 44 and 46 preferably consist of cadmium-platedcopper screens having an electrodeposited zinc coating thereon. In placeof the metal screens, cadmium-plated porous metal plaques or felt metalmay be used as a base for the zinc coatings. All four nickel oxideelectrodes 48 are double-faced electrodes. Conventional nickel oxideelectrodes of pocket-type or preferably sintered plaque construction aresuitably used. As shown, all of the zinc electrodes 44 and 46 areconnected in parallel; similarly, all of the nickel oxide electrodes 48are connected in parallel. Eight rotating apertured shutters 50 areshown, a shutter being interposed between each electrode pair. Theseshutters consist of thin apertured disks, suitably of nylon,polystyrene, or polypropylene, and are of the same diameter or slightlylarger than that of the electrodes. Shutters 50 are fixedly attached toa rotatable shaft 52 which rotates in a sleeve 54 and a shaft bearingand seal 56. A fixed-speed motor (not shown) is used to rotate theshaft. The motor suitably obtains its power from the battery whenseveral of the modules shown are connected in series to provide therequired voltage. Various insulated plastic spacers 58 are part of thestructure of the zinc electrodes 44 and 46 so as to maintain the desiredspaced-apart relationship between the zinc electrodes and the shutters,thereby preventing contact between these cell compo nents. The cell isfilled with a conventional alkaline electrolyte 59, suitably 35 wtpercent KOH containing zinc oxide dissolved therein.

The module shown in FIG. 2 corresponds to eight parallel-connectedelectrode pairs. Such a cell module is capable of providing 456 watts ofpower based on an obtained power of 57 watts per electrode pair forelectrodes having an area of about 400 square centimeters. For use in ahybrid vehicle, 120 of themodules shown are connected in series toprovidean open-circuit voltage of 210 volts and an operating voltage of156 volts at maximum load, to give a battery having 55 kilowatts ofpower. This power is obtained using individual electrodes having an areaof 400 square centimeters and an obtained current density of 0.1 1 ampsper square centimeter at 1.3 volts fora cell module having eightelectrode pairs, thereby providing 458 watts per cell module. Such azinc-nickel oxide battery assembly is capable of providing 55 kilowattsof power at power densi* ties of 135 watts per pound and 5.3 watts percubic inch, for a cycle life of many thousands of cycles underconditions of rapid cycling of the order of several seconds or minutes.Only approximately 300 cycles are now obtainable with the best of theconventional zincnickel oxide batteries and at considerably lower powerdensities.

The following examples, which are illustrative only and not to beconstrued as limiting the invention, describe the operation of azincnickel oxide cell containing a rotatable shutter interposed betweenthe electrodes.

EXAMPLE 1 A zinc-nickel oxide cell having a structure essentiallysimilar to that shown in FIG. 1 was utilized. The electrodes and theshutter separator were of circular cross section and had a diameter ofabout 6 inches. The zinc electrode consisted of a cadmium-plated copperscreen .having a zinc layer electrodeposited thereon. The

area of the disk being about equal to that of the masked area.

The cell was operated for more than 500 cycles under varying conditionsof charge and discharge. The shutter was rotated by a 56 rpm motorduring most of the time. For the greater part of the period, chargingwas conducted at 8 amp (44 ma/cm and discharge was at an average of9 amp(50 ma/cm) to nearly complete discharge. A quantity of Zinc equivalentto about 1.5 amp-hr was deposited during charge. The cell showedessentially no loss in capacity or voltage during the entire period.

After about 490 cycles, a 96 rpm motor was substituted with essentiallyno change in performance. However, when a lower speed 23 rpm motor wassubstituted, it was noted that for the particular shutter configurationused, a nonadherent zinc deposit was formed and the cell capacitydropped to a small fraction of its former value. Similarly, stopping theshutter rotation completely also destroyed the operability of the cellwithin a few cycles. In other experiments, a shutter was used having anopen area of about 70 percent of the total area. The shutter was rotatedat 96 rpm with good results being obtained. Again, stopping the rotationof the shutter destroyed the operability of the cell within a fewcycles.

High power output was obtained with the cell, it being possible todeliver 16 amps (89 ma/cm over a period of minutes before the cellvoltage dropped to 1.3 volts from an open-circuit voltage of about 1.75volts. The average voltage was about 1.5 volts for this period (24watts).

EXAMPLE 2 The cell shown in Example 1 was cycled by alternately chargingand discharging it at a rate of 12 minutes at 4 amp, the shutter beingrotated at 56 rpm. The cell was found capable of operation for more than3,000 cycles. Periodically the cell was tested on discharge under highpower output (18 amp) for a oneminute period. The cell voltage was1.35-1.40 volts at the beginning and 1.25-1.13 volts at the end of theoneminute period. No significant change was observed in either thecycling performance level or the high-output performance level for theentire cycling period of the cell. It was found that where somedepletion of the zinc concentration of the electrolyte occurred aftercontinued cycling, this concentration level could be restored bydischarging the cell at a 1.5 amp rate for about 90 minutes to removethe excess amount of zinc deposited on the zinc electrode. Regularcycling could be immediately resumed after completion of this specialdischarge cycle.

EXAMPLE 3 After the cell shown in Example 2 had completed 3,750 cycles,it was switched to a new duty cycle which simulated the cyclingrequirements for a hybrid automotive vehicle. This duty cycle involvedextremely rapid cycling of about 42 seconds for the entirechargedischarge cycle, with repeated charge-discharge cycling at low tomoderate current densities interspersed with periodic charge-dischargecycling at high current densities. Thus the cell provided l.85 volts ata discharge rate of 2.5 amp (l3 ma/cm) and 1.4 volts at 19 amp discharge(lOS ma/cm). More than 25,000 such duty cycles have been completed withno change occurring in the high power output of the cell.

EXAMPLE 4 In another cycling run, a cell similar to that of Example lwas designed for 20 watts maximum output. The cell was cycled inhalf-hour steps at 7 watts with occasional cycles at the maximum output.The chargedischarge cycle used was selected as typical of those to beexpected in a typical hybrid vehicle application. The cell was operatedwithout interruption in excess of 1,800 continuous charge-dischargecycles without degradation.

While the electrically regenerable electrochemical system of the presentinvention using a rotating apertured shutter is preferably utilized forelectrically regenerable zinc-nickel oxide or zinc-air batteries, it mayalso be employed advantageously in other batteries and for otherelectrochemical applications. For example, where the negative electrodeof the cell or battery is a zinc electrode, positive counterelectrodesthat are more electropositive than zinc may be used in place of nickeloxide or oxygen, e.g., silver oxide, manganese dioxide, and mercuricoxide. Conversely, batteries may be made using any of these positiveelectrode materials wherein other negative electrode materials areutilized in place of zinc. It is only necessary that the positiveelectrode material selected be electrochemically reactive, compatiblewith the electrolyte, and more electronegative than the counterelectrodeused. Such materials include lead, tin, iron, and cadmium for use inaqueous electrolytes, and aluminum and magnesium for use in nonaqueouselectrolyte systems. From the standpoint of cost, capacity, andconvenience, zinc is the preferred material for the negative electrode,and the improved battery of this invention using a rotating aperturedshutter between the fixed negative and positive electrodes has beendescribed therefore with reference to the use of a zinc negativeelectrode in conjunction with a nickel oxide counterelectrode.

The described cells will operate with conventional electrolytes, whichinclude the alkaline materials such as sodium hydroxide, potassiumhydroxide, mixtures of potassium and rubidium hydroxides, and the like.For certain applications, depending upon the nature of the electrode andcounterelectrode materials, an acid electrolyte, including sulfuric andphosphoric acids, can also be employed.

It will of course be realized that the construction of the nonconductivemovable shutter means disposed in the electrolyte between the negativeand the positive electrodes may be widely varied depending upon theparticular means utilized for moving the shutter means as well as therate at which the shutter means is moved. However, the basic requirementfor the shutter means is that it function so as to provide suitablystirring and circulation of the electrolyte and also provide amechanical equivalent of pulsed charging. Thereby uniform and adherentdeposits are formed in the electrodes. While the current take-off fromthe cell has not been shown other than schematically in the drawings,this may be any conventional means accessible by connection through thecasing or to the tabs emerging from the casing. Such means will bereadily apparent to those skilled in the art. it will thus be readilyapparent that within the scope and spirit of the present invention, itmay be practiced otherwise than as specifically illustrated anddescribed herein.

l claim:

1. An electrically regenerable electrochemical system comprisingspaced-apart negative and positive electrodes in cooperative relationwith an electrolyte, and nonconductive movable shutter means having anopen area of 30 to 80 per cent of the total area of the shutter means,said shutter means being disposed in said electrolyte between saidnegative and positive electrodes so that the continuous movement betweensaid electrodes of the open and closed areas of said movable shuttermeans exposes at least a portion of a facing surface of at least one ofsaid electrodes to alter nating periods of maximum and minimum currentdensity during operation of said system.

2. The electrochemical system according to claim 1 wherein the activematerial of said negative electrode is selected from lead, zinc, iron,cadmium and tin and at least a portion of a facing surface of saidnegative electrode is exposed to said alternating periods of maximum andminimum current density during operation of said system.

3. The electrochemical system according to claim 2 wherein the activematerial of said negative electrode is zinc and the active material ofsaid positive electrode is nickel oxide.

4. The electrochemical system according to claim 1 wherein said shuttermeans comprises a rotatable aper tured disk wherein the area of the openportion of the apertured disk constitutes between. 30 and per cent ofthe total disk area.

5. The electrochemical system according to claim 4 wherein the apertureddisk is rotatable at a speed between 20 and and 200 rpm, an increasingapertured area being correlated with a decreasing speed of rotation sothat the resultant alternating periods of maximum and minimum currentdensity are between 20 and 200 cycles per second.

6. The electrochemical system according to claim 4 wherein the openportion of the apertured disk consists of from two to ten symmetricallydisposed sectors.

7. The electrochemical system according to claim 6 wherein the activematerial of said negative electrode is zinc and the active material ofsaid positive electrode is nickel oxide and said disk is disposed incooperative relation with at least said negative: electrode so that atleast a portion of a facing surface thereof is exposed to saidalternating periods of maximum and minimum current density duringoperation of said system to form adherent zinc deposits so that saidsystem is capable of undergoing long cycle life at high power densitiesunder conditions of rapid cycling.

2. The electrochemical system according to claim 1 wherein the activematerial of said negative electrode is selected from lead, zinc, iron,cadmium and tin and at least a portion of a facing surface of saidnegative electrode is exposed to said alternating periods of maximum andminimum current density during operation of said system.
 3. Theelectrochemical system according to claim 2 wherein the active materialof said negative electrode is zinc and the active material of saidpositive electrode is nickel oxide.
 4. The electrochemical systemaccording to claim 1 wherein said shutter means comprises a rotatableapertured disk wherein the area of the open portion of the apertureddisk constitutes between 30 and 80 per cent of the total disk area. 5.The electrochemical system according to claim 4 wherein the apertureddisk is rotatable at a speed between 20 and and 200 rpm, an increasingapertured area being correlated with a decreasing speed of rotation sothat the resultant alternating periods of maximum and minimum currentdensity are between 20 and 200 cycles per second.
 6. The electrochemicalsystem according to claim 4 wherein the open portion of the apertureddisk consists of from two to ten symmetrically disposed sectors.
 7. Theelectrochemical system according to claim 6 wherein the active materialof said negative electrode is zinc and the active material of saidpositive electrode is nickel oxide and said disk is disposed incooperative relation with at least said negative electrode so that atleast a portion of a facing surface thereof is exposed to saidalternating periods of maximum and minimum current density duringoperation of said system to form adherent zinc deposits so that saidsystem is capable of undergoing long cycle life at high power densitiesunder conditions of rapid cycling.